Threat and Timing your Heals:

When a tank runs into the battle make sure to let him get aggro on all of the monsters in the engaged fight. Healing a tank right as he/she goes into battle is a sure fire way to pull aggro. If the tank fails to do so let them know! Communication will alleviate many of the problems that you'll encounter.


Making your Character and Stats:

Healers should focus on the following stats:
DET >> MIND >> PIETY >> VIT

Determination - Give your character higher spell damage and healing power (useful for when questing, etc.)

Mind - Direct healing bonus (This is especially important for ACNs since they don't get as much mind as CNJs)

Piety - More mana means more heals! It could be argued that this is a better stat for CNJs since they don't have access to a mana regeneration ability but in the end it's up to you what you'd rather have.

Vit - In the case that a monster gets on you it helps to have a fair amount of HP to take a few hits.